Various techniques are known for coating substrates with thin layers of polymeric materials. In general, the known techniques can be predominantly divided into three groups, (1) liquid coating methods, (2) gas-phase coating methods, and (3) monomer vapor coating methods. As discussed below, some of these methods have been used to coat articles that have very small surface feature profiles.
Liquid Coating Methods
Liquid coating methods generally involve applying a solution or dispersion of a polymer onto a substrate or involve applying a liquid reactive material onto the substrate. Polymer or pre-polymer application is generally followed by evaporating the solvent (in the case of materials applied from a solution or dispersion) and/or hardening or curing to form a polymer coating. Liquid coating methods include the techniques commonly known as knife, bar, slot, slide, die, roll, or gravure coating. Coating quality generally depends on mixture uniformity, the quality of the deposited liquid layer, and the process used to dry or cure the liquid layer. If a solvent is used, it can be evaporated from the mixture to form a solid coating. The evaporation step, however, commonly requires significant energy and process time to ensure that the solvent is disposed of in an environmentally-sound manner. During the evaporation step, localized factors—which include viscosity, surface tension, compositional uniformity, and diffusion coefficients—can affect the quality of the final polymer coating.
Liquid coating techniques can be used to coat materials onto substrates that have small surface feature profiles. For example, U.S. Pat. No. 5,812,317 discloses applying a solution of prepolymer components and a silane coupling agent onto the protruding portions of partially embedded microspheres. And U.S. Pat. No. 4,648,932 discloses extruding a liquid resin onto partially embedded microspheres. As another example, U.S. Pat. No. 5,674,592 discloses forming a self-assembled-monolayer coating of octadecyl mercaptan and a partially fluorinated mercaptan (namely, C8F17(CH2)11SH) from a solvent onto a surface that has small surface feature profiles.
Gas-phase Coating Methods
Gas-phase coating techniques generally include the methods commonly known as physical vapor deposition (PVD), chemical vapor deposition (CVD), and plasma deposition. These techniques commonly involve generating a gas-phase coating material that condenses onto or reacts with a substrate surface. The methods are typically suitable for coating films, foils, and papers in roll form, as well as coating three-dimensional objects. Various gas-phase deposition methods are described in “Thin-Films: Film. Formation Techniques,” Encyclopedia of Chemical Technology, 4th ed., vol. 23 (New York, 1997), pp. 1040-76.
PVD is a vacuum process where the coating material is vaporized by evaporation, by sublimation, or by bombardment with energetic ions from a plasma (sputtering). The vaporized material condenses to form a solid film on the substrate. The deposited material, however, is generally metallic or ceramic in nature (see Encyclopedia of Chemical Technology as cited above). U.S. Pat. No. 5,342,477 discloses using a PVD process to deposit a metal on a substrate that has small surface feature profiles. A PVD process has also been used to sublimate and deposit organic materials such as perylene dye molecules onto substrates that have small surface features, as disclosed in U.S. Pat. No. 5,879,828.
CVD processes involve reacting two or more gas-phase species (precursors) to form solid metallic and/or ceramic coatings on a surface (see Encyclopedia of Chemical Technology as cited above). In a high-temperature CVD method, the reactions occur on surfaces that can be heated at 300° C. to 1000° C. or more, and thus the substrates are limited to materials that can withstand relatively high temperatures. In a plasma-enhanced CVD method, the reactions are activated by a plasma, and therefore the substrate temperature can be significantly lower. CVD processing can be used to form inorganic coatings on structured surfaces. For example, U.S. Pat. No. 5,559,634 teaches the use of CVD processing to form thin, transparent coatings of ceramic materials on structured surfaces for optical applications.
Plasma deposition, also known as plasma polymerization, is analogous to plasma-enhanced CVD, except that the precursor materials and the deposited coatings are typically organic in nature. The plasma significantly breaks up the precursor molecules into a distribution of molecular fragments and atoms that randomly recombine on a surface to generate a solid coating (see Encyclopedia of Chemical Technology as cited above). A characteristic of a plasma-deposited coating is the presence of a wide range of functional groups, including many types of functional groups not contained in the precursor molecules. Plasma-deposited coatings generally lack the repeat-unit structure of conventional polymers, and they generally do not resemble linear, branched, or conventional crosslinked polymers and copolymers. Plasma deposition techniques can be used to coat structured surfaces. For example, U.S. Pat. No. 5,116,460 teaches the use of plasma deposition to form coatings of plasma-polymerized fluorocarbon gases onto etched silicon dioxide surfaces during semiconductor device fabrication.
Monomer Vapor Coating Methods
Monomer vapor coating methods may be described as a hybrid of the liquid and gas phase coating methods. Monomer vapor coating methods generally involve condensing a liquid coating out of a gas-phase and subsequently solidifying or curing it on the substrate. The liquid coating generally can be deposited with high uniformity and can be quickly polymerized to form a high quality solid coating. The coating material is often comprised of radiation-curable monomers. Electron-beam or ultraviolet irradiation is frequently used in the curing (see, for example, U.S. Pat. No. 5,395,644). The liquid nature of the initial deposit makes monomer vapor coatings generally smoother than the substrate. These coatings therefore can be used as a smoothing layer to reduce the roughness of a substrate (see, for example, J. D. Affinito et al., “Polymer/Polymer, Polymer/Oxide, and Polymer/Metal Vacuum Deposited Interference Filters”, Proceedings of the 10th International Conference on Vacuum Web Coating, pp. 207-20 (1996)).